Coating composition for metals



United States Patent 2,798,015 COATING COMPOSITION FOR-METALS David K. Lesser, Wilmington, Del., assignor to Hercules Powder Company, Wilmington, DeL, a corporation of Delaware No Drawing. ApplicationJune 7,1954, Serial N0. 435,081

1 Claim. (Cl. 148--6.16)

This invention relates to a'coating'composition for metals and more particularly to a coating composition which has improved adhesion to metals, the ability to inhibit the corrosion of metals and wherein'a mixture of ethyl cellulose and polyvinyl butyral resin is the film-forming ingredient.

The chemical treatment of metal surfaces to render themresistant to corrosion and provide a suitable surface to which a protective coating will adhere has been the object of considerable investigation. Such chemical treatment is commonly accomplished by the use of wash primers which are organic coatings applied in very thin films, in the order of 0.3 mil, as a pretreatment conditioner for subsequent painting. In view of the thinness of the coating, it is quite important that the film produced is uniform and continuous. If a nonuniform and discontinuous film is produced due to contamination or lack of good film properties for any reason, the composition has little value or at best is inferior as a wash primer.

Three of the most difficult properties to obtain in organic coatings for metal are adequate adhesion, impact resistance, and prevention of under film corrosion of the metal. Numerous treatments and coatings have been developed in the past in an endeavor to obtain these desired properties. They include phosphoric acidtreatrnent (e. g., Bonderizing), zinc chromate primers, red lead primers, and phenolic resin primers.

Wash primers, which represent both a metal treatment and an organic coating, are one of the most recent methods for improving metal coatings. The earliest wash primer, a two-package system, was called the WP1 type. This system employed a dispersion of zinc chromate, polyvinyl butyral, and solvents in one package, and a phosphoric acid diluent in the other. This wash primer had poor can stability (about eight hours), was expensive, and its film was prone to blister under conditions of high humidity. An improved wash primer called the chromicacid type was later developed. This primer consisted of an alcoholic solution of polyvinyl butyral reacted at elevated temperature with phosphoric and chromic acids. This later chromic acid wash primer has advantages over WP-l primers in that can stability was improved (over one year) as well as resistance to humidity. The major disadvantages to the chromic acid wash primers now commercially available are the high cost of raw materials, the hazardous chernicals used in their manufacture and their susceptibility to contamination which usually results in poor performance. Chromic phosphate wash primers which are a dispersion of chromic phosphate pigment, polyvinyl butyral and solvent which is diluted with a phosphoric acid solution were a further advance which incorporated the ease of manufacture usually associated with WP-l wash primers, and the performance advantages of the chromic-acid type.

With these disadvantages in view, it is, therefore, a primary object of this invention to provide an improved corrosion inhibiting coating or wash primer of the chromic-phosphate type for metals including steel, aluminum,

and the like wherein relatively low'cost raw materials are utilized which are readily available and which may be reacted at room temperature with satisfactory results-to produce a wash primer having good properties in respect to impact resistance, scratch adhesion of subsequent coatings and resistance to contamination, which properties additionally contribute to improving the corrosion resistance of the wash primer.

Other objects of the invention will appear hereinafter; the novel features and combinations being set'for'th in the appended claims.

According to this invention, there is provided a coating composition, characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals, comprising a dispersion product of phosphoric acid solution in admixture with the product obtained by milling chromic phosphate with an' organic solvent blend of -ethyl cellulose and polyvinyl butyral resin. I

The following'exatnple is illustrative of the invention wherein all parts given are parts by weight.

Example A mixture of 4.5 parts of Vinylite XYHL, 4.5' parts of N-lO ethyl cellulose, 9.0 parts of chromic phosphate, 1.4 parts of magnesiumsilicate, 54.5 parts of 3A ethyl alcohol, and 16.1 parts of methyl isobutyl ketone was placed in a pebble mill and ground until a smooth dispersion was obtained. An acid diluent of 1.8 parts of phosphoric acid, 1.8 parts of water, and 6.4 parts of 3A ethyl alcohol was then added to the dispersion and suitably mixed. The resulting reaction composition had a medium green color, a solids content of 19.4%, and a viscosity of 91 seconds (Ford No. 4 cup).

Compositions in accordance with this invention which were prepared as set forth in the foregoing example were applied to metal panels. The panels tested were aluminum (3S) and cold rolled steel. The panels were first thoroughly cleaned in solvent, then one-half of each panel was dipped in the wash primer of this invention using a dip coater adjusted to provide a 0.3-0.5 mil dry film. One set of panels was topcoated the same day as dipping, another set of panels was topcoated after aging one week. and still another set of panels was topcoated after baking 15 minutes at 250 F. After the designated drying schedule, the entire panel was sprayed with three coats of nitrocellulose lacquer.

Impact resistance tests and scratch adhesion tests carried out on the various sets of panels showed generally an improvement in impact resistance and in scratch adhesion of the film applied over the wash primer of this invention in comparison with the film applied over the bare metal. Thus, in addition to the known corrosion resistance imparted by wash primers, wherein phosphoric acid and chromic phosphate mixtures have been disclosed as reacting with a metal surface to deposit a tightly adhering and insoluble layer of metal salt, there is provided an excellent film-forming constituent which is ethyl cellulose and polyvinyl butyral bound in some manner to form a tightly adhering complex of the whole composition to metals. The exact mechanism by which the compositions of this invention are bound to metals is not known. However, it has been observed physically that the ethyl cellulose-polyvinyl butyral film or complex thereof is smooth, uniform, continuous and adhesive which is a highly important requisite for a corrosion inhibiting coating which also serves as a ground or base coat for painting.

It is evident that there are several factors which will influence conditions for the most satisfactory operation of this invention, the actual limits of which can be determined only by a detailed study of each set of materials to be utilized in preparing the compositions and the types of metals to which the compositions are to be applied. For example, the following factors will affect the suitability of the compositions of this invention for certain specific application: composition of solvent blend, water content, ratio of chromic acid to phosphoric acid and acid concentration, optimum reaction time, type and grade of ethyl cellulose and polyvinyl butyral resin and type of metal to which the composition is to be applied.

Generally, however, it will be found that compositions prepared in proportions and using ingredients approximating those set forth in the example will usually give satisfactory results for general application to certain metals. In this respect, although 3A (methanol denatured) alcohol and methyl isobutyl ketone is preferred as set forth in the example, 23 (benzene denatured) or other alcohol or other suitable organic solvent may be used. Although N-lO (46.848.5% ethoxyl content and a viscosity of 10 centipoises in 5% concentration solution) ethyl cellulose is preferred as set forth in the example, organosoluble ethyl celluose of other ethoxyl content and viscosity may be used within practical limits. Similarly, although Vinylite XYHL (vinyl content 54.4, butyraldehyde 38.3, acetate 0.3 and hydroxyl 7.0% with intrinsic viscosity of 0.81) polyvinyl butyral resin is preferred as set forth in the example, other organosoluble vinyl butyral resins may be used within practical limits in combination with organosoluble ethyl cellulose for obtaining the coating composition of this invention. It has been found from test results that the product obtained from a 1:1 ratio of ethyl cellulose-polyvinyl butyral resin appears to approach a maximum in performance relative to impact resistance and scratch adhesion. Based on 1000 parts by weight of the total composition, the chromic phosphate, phosphoric acid, ethyl cellulose, and polyvinyl butyral resin may be varied within the following ranges: 1 to 60; 1 to 50; to 500; and 10 to 500, respectively. The concentration of the product of this invention based on the total Weight of the coating composition may be varied from about 3 to about by weight. Pigments and diluents such as magnesium silicate shown in the example may be added, if desired. Generally, however, the coating composition will be used on metals without addition of other materials since it is quite common practice to apply a protective coating over the primed metal base. Such protective coatings may be applied in the usual manner such as by dipping, spraying,

and brushing. However, the primer coating composition of this invention may be applied using the above conventional methods of application and suitably dried at room or elevated temperature. Additionally, the compositions may be applied to a metal base by dipping or immersion with or without residence time and then suitably dried.

Certain uses for the coating compositions of this invention reflect its advantages in giving a smooth, uniform and continuous film and include more particularly such uses where the conventional WP-l type wash primers are now being used such as marine and other product finishes where corrosion resistant coating systems are desired for salt water exposure.

What I claim and desire to protect by Letters Patent 1s:

A coating composition, characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals, consisting essentially of a dispersion product of a solution containing 1 to parts by weight of phosphoric acid in admixture with the product obtained by milling 1 to parts by weight of chromic phosphate with an organic solvent solution blend of 10 to 500 parts by weight of organo-soluble ethyl cellulose and 10 to 500 parts by weight of polyvinyl butyral resin, said organic solvent solution consisting of a mixture of lower aliphatic alcohols, ketones and water, the alcohols having 1 to 4 carbon atoms, the ketones being solvents for the phosphoric acid solution, the amounts of said alcohols, ketones and water being sufficient for said ethyl cellulose and polyvinyl butyral resin to dissolve in the alcohols and said phosphoric acid solution to dissolve in the ketones, the concentration of said dispersion product being from 3% to 20% by weight based on the total weight of said coating composition, said coating composition being prepared at about room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,254,263 Oeschger Jan. 22, 1918 1,514,494 Williams Nov. 4, 1924 2,293,558 Overholt Aug. 12, 1942 2,412,543 Tanner Dec. 10, 1946 2,469,318 Swan May 3, 1949 2,525,107 Whiting et al. Oct. 10, 1950 

